Self-Assembly Revolutionizes Metamaterial Manufacture

A new self-assembly technique could make optical metamaterials cheaply and easily for the first time

In recent years, we’ve heard much about metamaterials and their potential to steer light in all kinds of weird and wonderful ways. There’s no end of things you can build with this stuff: everything from illusion cloaks that make one object look like another to artificial black holes and multiverses.

But while theory races ahead, the materials scientists are puzzling over how to make this stuff cheaply and easily. Metamaterials consist of repeating patterns of elements that interact with an electromagnetic wave so as to control and distort its path

At millimeter and microwave wavelengths, these elements are things like wires and c-shaped pieces of metal called split-ring resonators. To make them into metamaterials, they have to be assembled into arrays with vital statistics of the order of the light they are intended to interact with. That can be done by hand or by conventional manufacturing techniques.

But at infrared and optical wavelengths, the creation and assembly of active electromagnetic elements is a major headache. The best attempts so far involve drilling holes into sheets of gold and silicon using ion beams. That’s slow and fiddly. Consequently, these manufacturing techniques are the major limiting factor in how optical metamaterials can be exploited.

Today, we learn of a promising new technique that has the potential to produce these kinds of materials on a truly industrial scale for the first time. “We show that we can circumvent this limitation,” say Kristof Lodewijks at the Interuniversity Microelectronics Center in Belgium and a few buddies.

The technique is surprisingly simple. These guys take a handful of polystyrene balls just a few hundred nanometers in diameter and spread them onto a flat gold surface where they naturally self assemble into a hexagonal array with few, if any, defects.

This array then serves as a template for the metamaterial. Using a series of steps in which the surface is covered in masks and then etched, the team create a thin slab consisting of a gold-silicon oxide-gold sandwich. However, this slab is punctured by a hexagonal array of holes that correspond to the position of the polystyrene beads.

This slab, say Lodewijks and co, is a negative refractive index material that operates in the near infrared region. And the operating wavelength is simple to change by altering the diameter of the beads, which in turn changes the size of the holes.

That’s clever, cheap and quick. It’s also a technique that could be easily scaled. So we may finally have a way to make negative index materials at optical frequencies on a decent scale.